Semiconductor wafers of silicon, gallium-arsenic, etc., are produced normally in their state of being relatively large in diameter, and the wafers are diced into chips, which are then transferred to the subsequent mounting stage. At this time, the semiconductor wafers are subjected in their state of being previously applied to an adhesive layer of an adhesive sheet to such stages as dicing, rinsing, drying, expanding, picking-up and mounting.
Adhesive sheets which have heretofore been used in the dicing stage of semiconductor wafers as mentioned above include those which comprise a substrate such as a vinyl chloride or polypropylene film or sheet and thereon an adhesive layer consisting essentially of such an adhesive as an acrylic adhesive or the like. In the adhesive sheets having an acrylic adhesive layer, however, there was involved such a problem that the chips were contaminated on the back side surface thereof with the adhesive which had adhered to and remained thereon.
In order to solve the problem mentioned above, there has been proposed a process where an adhesive applied to the surface of an adhesive sheet is minimized by coating a substrate of the adhesive sheet, not wholly but partly, with the adhesive. According to this process, however, there is brought about such a new problem that because of a decrease in adhesion strength between wafer chips and adhesive sheet, the wafer chips peel off from the adhesive sheet during stages subsequent to the dicing stage, such as rinsing, drying and expanding, though the amount of the adhesive used relative to the full number of the chips decreases and thereby lessens contamination of the back side surface of the chips with the adhesive to a certain extent.
Adhesive sheets which are intended for use in the processing stage of semiconductor wafer, from the dicing stage up to the pick-up stage, are desired to be such that the adhesive sheets have an adhesion force sufficient to retain wafer chips thereon in the course from the dicing stage up to the expanding stage, but in the pick-up stage, they only retain their adhesion force to such an extent that no adhesive sticks to the back side surface of picked-up wafer chips.
PCT/US80/00822 patent application discloses normally tacky and pressure-sensitive adhesive including oxirane rings and an ionic photoinitiator characterized in that the epoxy equivalent value of the adhesive is 400-900 and the ionic photoinitiator is selected from the group consisting of radiation-sensitive aromatic onium salts of Group Va or Vla; onium catalysts of Group Va, VI or VIIa, diaryl halonium salts containing Group Va metal hexafluorides and triaryl sulfonium complex salts and is present in an amount effective to promote the polymerization of oxirane rings, whereby said adhesive is readily detackifled by exposure to actinic radiation.
Further Japanese Patent Laid-Open Publication Nos. 196956/1985 and 223139/1985 disclose adhesive sheets which are alleged to satisfy the above-mentioned requirements, said adhesive sheets comprising a substrate having coated on the surface thereof an adhesive consisting essentially of a low molecular compound having in the molecule at least two photopolymerizable carbon-carbon double bonds and capable of exhibiting a three-dimensional network on irradiation of light. The low molecular compounds having in the molecule at least two photopolymerizable carbon-carbon double bonds as exemplified in the above-mentioned publications include trimethylolpropane triacrylate, tetramethylolmethane tetraacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, dipentaerythritol monohydroxy pentaacrylate, dipentaerythritol hexaacrylate, 1,4-butyleneglycol diacrylate, 1,6-hexanediol diacrylate, polyethylene glycol diacrylate, commercially available oligoester acrylates, etc.
We have found that the adhesive sheets comprising a substrate having coated thereon an adhesive layer consisting essentially of such low molecular compound having in the molecule at least two photopolymerizable carbon-carbon double bonds as exemplified above involve problems as mentioned below.
(1) Such an adhesive sheet as mentioned above can exhibit an excellent performance to a certain extent when a semiconductor wafer to be applied to the adhesive sheet has a fine and smooth surface like a mirror surface. If the wafer surface is rough, however, there is observed such a problem that even when irradiation of such radiation as ultraviolet ray is effected in the pick-up stage, the adhesion strength between the wafer chips and adhesive sheet does not decrease sufficiently and accordingly the adhesive sticks to and remains on the back side surface of the chips.
(2) Where a semiconductor wafer applied onto an adhesive layer of an adhesive sheet is diced into wafer chips and the diced chips are then picked up, the position of the chip has been detected by means of a photosensor. In the above-mentioned adhesive sheet, however, there is observed such a problem that because the adhesive sheet reflects the detection beam at the chip's position, the detection operation of the chip cannot be performed with satisfactory accuracy and a failure in the detection operation sometimes occurs.
(3) Where the wafer surface to which an adhesive layer of an adhesive sheet is applied is greyed or blackened for some reason or other, there is involved such a problem that even when the adhesive sheet is irradiated with radiation such as ultraviolet ray in the pick-up stage of the semiconductor wafer chips, the adhesion force of the portions of the adhesive layer of the adhesive sheet corresponding to the portions of the greyed or blackened portions of the wafer does not decrease sufficiently and accordingly the adhesive adheres to and remains on the back side surface of the wafer chips.
(4) In the adhesive sheet as mentioned above, there is observed such a problem that the adhesive force certainly decreases but not to the optimum level, thus the larger the size of the chips, the more difficult the pick-up operation becomes.
(5) In the above-mentioned observed such a problem that because such a general-purpose polymer sheet as of polyvinyl chloride or polypropylene is used as a substrate sheet in said adhesive sheets, a tensile force is applied to said adhesive sheets when wafers are applied onto said adhesive sheets or when the thus applied wafers are diced into chips, and the substrate sheet undergoes elongation after the completion of the dicing stage of the wafers, whereby a deflection occurs in the adhesive sheets and the thus deflected adhesive sheets cannot be received by a wafer box for transferring said adhesive sheet to the subsequent processing operation, or the received wafers mutually contact with one another in the wafer box. Furthermore, there is also observed such a problem that when the adhesive sheets, after the completion of the dicing operation of the wafers applied onto said adhesive sheets, are subjected to irradiation with radiation such as ultraviolet ray, an elongation or deflection occurs afresh in the irradiated adhesive sheets, or such elongation or deflection which has occurred once in the adhesive sheets in the dicing stage of the wafers as mentioned previously is sometimes retained, as it was, in the irradiated adhesive sheets, and hence said irradiated adhesive sheets cannot be received by a wafer box for transferring said adhesive sheets to the subsequent pick-up stage, or the received wafers mutually contact with one another in the wafer box.